Human liver cytochrome P450s collectively metabolize a vast multitude of drugs and non-drug xenobiotics including environmental toxins, mutagens and carcinogen as well as endobiotics such as steroids, prostaglandins and fatty acids. These are about twelve major isoforms of cytochrome P450 responsible for this metabolism. The precise measurement of the tissue content and metabolic role of each P450 isoform responsible for the metabolism of a substrate requires reagents of high specificity which inhibit the enzyme activity of each P450 isoform. Inhibitory monoclonal antibodies (MAb) are reagents par excellence for the highly specific quantification of the metabolic activity of the multiple isoforms of cytochrome P450.